A switching power supply can be implemented using conventional circuit topologies such as, for example, a push-pull converter, a half-bridge converter, a pulse width modulated full-bridge converter, or a phase modulated full-bridge converter. For each of these converter circuits, if the load current grows too large, a control circuit responds by changing the state of one or more switches in the converter circuit to limit the load current. Conventional control circuits sense only the magnitude of the load current and, if determined to be too large, terminate the active current driving pulse or shut down the converter circuit altogether.
With these conventional control circuits, however, the high current condition may persist after pulse termination due to a lack of load voltage to drive the current down, overheating the circuit elements. This commonly occurs in plasma generation situations due to the low impedance of a plasma after ignition. Or, the delay in sensing the over current condition in the load and then changing the state of the converter circuit's switches may be too long. Accordingly, in many converter circuits, the state of a switch may be changed too late (i.e., during the following opposite polarity portion of the switching cycle). This can slow, rather than quicken, the correction of the load current. In particular, the load current may fail to return to zero and instead continue to have a single polarity over several switching cycles. This can interrupt the power conversion process or destroy the converter circuit's elements. Moreover, the resulting load current waveform may have multiple transitions, causing excessive dissipation in the switching elements.
It is therefore an object of this invention to provide systems and methods for controlling a switching power supply. Another object of the invention is to provide systems and methods for controlling a power supply to rapidly return to a normal operating condition.